What is the round thing (usually called a "phase plug") suspended over the center of a tweeter for?

Question

Also why do some phase plugs have a hole in the center and some don't?

Answer 1

They are there to correct the phasing of the sound waves, so that they begin in synchronization with each other and do not cancel each other out, which leads to poor quality sound. The hole depends on the design of the particular speaker. Engineers must calculate the physical modes that the speakers vibrate in, and the phase plugs will help to avoid 'muddy' acoustics. They are slightly different from speaker to speaker.

8 DEC 06, 1901 hrs, GMT.